Servo, variable frequency, stepper and other similar types of electric motors and actuators (“pulsed signal devices”) are driven by pulsed signals provided by special controllers, sometimes called amplifiers. The examples described further in this disclosure for simplicity and clarity relate to servo motors, however the same approach is applicable to other relevant types of motors and actuators.
Servo and similar motors are driven by pulsed signals where the properties of said signals determine rotation and position of the rotor of said motors. Most such drive signals utilize pulse-width modulation (PWM) where the drive signals are pulses with certain phase and frequency. FIG. 1A shows the typical application of a servo motor 10, a controller 20 and wires 30 that connect said amplifier to said motor. Servo controller is powered by either AC or DC voltage, shown as 40 in FIG. 1A. A typical servo motor is powered by three-phase pulsed drive signals generated by said servo controller. Variations in pulse width and phase shift of pulses determine rotor's position and rotation speed. A typical waveform of one phase of the drive signal of a servo motor is shown as 50 in FIG. 1B. A typical electrical noise generated by servo amplifier and transmitted into power line is shown as 60 in FIG. 1C.
A pulsed signal device, such as a servo, a variable frequency, a stepper and other similar types of electric motors and actuators, is ubiquitous in industrial automation since they allow for precise and controlled movement of equipment parts. One of main challenges in the use of pulsed signal devices is artifacts of drive signals. Specifically, the pulses that drive the motor having a typical repetition rate of 8 kHz to 20 kHz and may have very short rise and fall times as shown in FIG. 2A. The rise and fall times can be as short as tens of nanoseconds. Because drive voltage is high (the most common voltage is 200V with the range typically between 100 and 400V), dV/dt, or the rate of change of voltage over time can be very substantial. This creates several problems listed below in no particular order of significance.
First, the transmission of drive pulses from the controller to the motor is done via regular power cables with little or no consideration for high speed signals where high speed refers not to the frequency of the drive signal but rather to the spectrum of the rise and fall times of that signal. The output impedance of the servo controller typically does not match the impedance of the windings of the motor. All the above transforms sharp edges of the drive pulses into ringing signals with peaks higher than the amplitude of the pulses. FIG. 2b shows a display 120 of a typical ringing of the drive pulse. As seen, drive pulses 100 (only one phase shown for clarity) have repetition frequency of 12 kHz and amplitude of 200V in this case. As described above the rise and front edges of the drive pulses have overshoots and ringing shown as 110. The amplitude of the overshoot here is ˜80V over the normal amplitude of the drive pulses. Signal 120 of FIG. 2B shows another example of artifact of sharp rise and fall edges where the overshoot reaches at least three times the normal drive signal amplitude. This causes reliability issues with the motor as described further.
Second, due to parasitic capacitive coupling between the windings of the motor and the rotor (described in “Capacitively coupled discharging currents in bearings of induction motor fed from PWM (pulsewidth modulation) inverters” by Adam Kempski, Journal of Electrostatics, 10/22/12 and in “Effect of PWM Inverters on AC Motor Bearing Currents and Shaft Voltages” by Jay Erdman, et. al., IEEE APEC Conference Dallas. Tex. March, 1995) sharp rise and fall times of drive pulses cause voltage on rotor and subsequent current to ground in the motor's bearing damaging and eventually destroying them. FIGS. 3A-3C illustrate this phenomenon with the drawings from the above and other sources. FIG. 3A depicts electric motor 150 with stator windings 160 and rotor 170. Parasitic capacitive coupling 180 between said rotor and stator acts as a conduit for high-frequency currents caused by sharp edges of drive pulses. This current 190 flows via ball bearings 200 to the motor frame and back to the servo controller. FIG. 3B shows typical voltage and current on said rotor. Although the current doesn't seem to be excessively high, due to its repetition rate it eventually causes damage to the ball bearings as depicted in FIG. 3C.
Third, the current in the ground circuits resulting from sharp rise and fall times of the drive pulses causes electrical overstress (EOS) to the sensitive components that may be processed by the equipment driven by servo motors (Electrical Overstress (EOS): Devices, Circuits and Systems by Steven Voldman, Wiley, 2014).
Fourth, the drive circuit of the servo controller generates significant electrical noise into the power line (typically AC power line) which causes serious interference problems with other equipment and issues with EMC (electromagnetic compliance).
Various known methods are used for mitigation of the above problems. For the noise problems between the servo controller and the motor, various servo motor filter solutions exist. For example, Copley Control's Xenus XTL-FA-01 edge filter provides an increase of dV/dt of the rise and fall times of the drive pulses on servo motors which reduces coupling voltages inside the motor and resulting bearing current. As another example, the TCI V1k motor protection filters (http://www.transcoil.com/Products/V1k.htm) is another solution to increasing dV/dt. There are other similar solutions on the market. While these existing filters offer an increase of dV/dt which decreases current through the motor bearings, they do not address reduction of this current directly.
For the mitigation of noise problems on supply AC lines and EMI problems, servo controller manufacturers recommend use of power line AC EMI filters as shown, for example, in Panasonic' Instruction Manual AC Servo Motor and Driver MINAS A4P Series, pages 28, 29 and similar documents.
The existing solution thus requires two filters—one between the servo controller and the motor and another filter between the servo controller and the power line feeding it as shown in FIG. 4A. This typical two filter approach creates a space problem because space in the tools that use servo motors is very limited as shown in typical installations as depicted in FIGS. 4A and 4A. In addition, the cost of the two filters in the two filter configuration may be high.
Thus, it is desirable to provide a method and a device to a) directly reduce current through the motor's bearings and to reduce physical space and cost associated with complete filtering of noise related to servo motor and amplifier operation.